The present invention relates to an optical wavelength conversion module comprising a fiber-type optical wavelength converter in which a fundamental wave of semiconductor laser light is incident to a core, and a second harmonic thereof is emitted as converted light.
A non-linear optical effect is a phenomenon that when light is incident to a medium, there arises polarization proportional to higher order terms, quadratic and more, of an electric field of the light, and this phenomenon produces a second harmonic, a sum frequency, a difference frequency, and so on.
The material producing such a phenomenon is called a non-linear optical material, and as the material, inorganic materials such as KH.sub.2 PO.sub.4, LiNbO.sub.3, LiTaO.sub.3, etc. have been well known. Recently, however, organic materials represented by 2- methyl-4-nitrileaniline (MNA), 4-dimethylamino-3-acetoamidonitrobenzene (DAN), and 3,5-dimethyl-1-(4-nitrophenyl)pyrazole (DMNP) has also attracted the attention because of their large non-linear optical constants.
Recently, studies for applying such a non-linear optical material to a fiber-type optical wavelength converter for halving the wavelength from a semiconductor laser light source using inter-band transition of a semiconductor are performed eagerly, and converters using LiNbO.sub.3 or DMNP have been known.
In such a converter, in order to produce a second harmonic or the like with a high efficiency, it is important to design the converter to confine a fundamental wave with a high energy density, and ensure the interaction length between the fundamental wave and the higher harmonic. Therefore, either a core or a clad of an optical fiber is formed of single crystal or polycrystal of a non-linear optical material, and an amorphous material such as glass is used as either the clad or the core, so that a fundamental wave is guided into the core to thereby obtain a high conversion efficiency. FIG. 3 shows the state where a fundamental wave 6 having passed a core 41 of a fiber-type optical wavelength converter 4 is emitted after converted into a second harmonic 5.
In a fiber-type optical wavelength converter, it is also necessary to make the propagation rate of a fundamental wave coincident with that of a produced second harmonic, that is, to make phase matching between the fundamental wave and the second harmonic. The phase matching implicates that, as shown in FIG. 4, on the assumption that a second harmonic is produced at a point A from light propagating through the core 41 and leaks out to a clad 42 at an angle .alpha., in the case where the direction .alpha. at a point B after elaspe of a unit time is coincident to the equiphase plane of the first-mentioned second harmonic, a second harmonic is radiated (Cherenkov Radiation) in the direction of this angle .alpha.. Let the refractive index of the clad 42 to a fundamental wave be n.sub.S (.omega.), the refractive index of the core 41 be n.sub.G (.omega.), and the refractive index of the clad 42 to a second harmonic be n.sub.S (2.omega.), phase matching is made automatically to thereby make Cherenkov Radiation possible only if the following condition is satisfied. EQU n.sub.S (2.omega.)&gt;n.sub.G (.omega.)&gt;n.sub.S (.omega.)
For example, the investigation to use an optical wavelength conversion module comprising such a fiber-type optical wavelength converter for reading an optical disk is being advanced. To this end, an optical disk light source for generating a fundamental wave must be small, and therefore a semiconductor laser is employed.
In this case, noises of semiconductor laser light is amplified because the intensity of a second harmonic is proportional to the square of that of a fundamental wave, so that a high S/N ratio is required in the semiconductor laser light.
In the case of multi-longitudinal modes (spectra) of a semiconductor laser light, a sum frequency wave of two longitudinal modes is produced together with a second harmonic in a fiber-type optical wavelength converter, and the intensity of the sum frequency wave fluctuates as time goes. It has been therefore considered that it is preferable and necessary to use a single longitudinal mode of semiconductor laser oscillation in order to obtain light of narrow wavelength range.
If the number of longitudinal modes of semiconductor laser light is made one, however, when the above-mentioned semiconductor laser light is irradiated to a fiber-type optical wavelength converter, the semiconductor laser light source is badly affected by so-called "return" of light which is a phenomenon that the reflected light returns to the semiconductor laser light source. It is said that the effect appears not only in the case of strong reflected light fed back to a laser aperture, but also in the case of very weak one about 10.sup.-7 of light emitted from the semiconductor laser. The distance between the semiconductor laser light source and a reflection point for the effect to appear is in a wide range from a .mu.m order to a km order ("O plus E", Feb. 1984, pp.89 to 97).
Mode jump is induced in the semiconductor laser by this return phenomenon, and many noise components are produced in output light. Consequently the S/N ratio of a second harmonic outputted from the fiber-type optical wavelength converter deteriorates.